It's important to keep the pawn on g4 in order to restrict the c2-bishop.
9... b6b3 Black has three equally good moves: 9...Qxb3, 9...Be6 and 9...Be4. 10. a2b3  10... f5c2 A critical move, but I wonder if retreating the bishop to e6 wasn't a more practical choice. 11. c1f4  11... h7h5 Black fights to give the light-squared bishop a way back.